An approximate method based on local eld perturbations caused by abrupt transitions of the surface pro le is described, which enables rapid approximate calculation of di raction patterns of di ractive elements in the non-paraxial domain of di ractive optics. In this domain the simple complexamplitude transmittance model based on scalar di raction theory is no longer valid and rigorous computations are exceedingly time-consuming. Comparisons with rigorous di raction theory show that the method is accurate for binary and multilevel pro les provided that the minimum feature size is larger than approximately one optical wavelength. The method is applied to the evaluation of the uniformity error of binary, four-level, and analogue array illuminators.
IntroductionIt has become customary to divide the domain of di ractive optics into two subdomains according to the analysis methods involved, even though any such division is necessarily an oversimpli cation [1]. If all features in the surface-relief pro le are much larger than the optical wavelength ¶ and the optical signal generated by the element is therefore of paraxial nature, one speaks of the paraxial domain and makes use of the well-known complex-amplitude transmittance approach to model and design di ractive elements. If, on the other hand, wavelength-scale features are present in the di ractive structure and the signal is non-paraxial, one speaks of the non-paraxial domain and resorts to rigorous di raction theory. In the paraxial domain the modelling and design are relatively simple tasks, but in the non-paraxial domain major computational di culties are faced in a large majority of problems [2]. Therefore methods, which are more accurate than the complex-amplitude transmittance approach but computationally less demanding than implementations of rigorous di raction theory are of considerable value. In this paper we consider in detail one such method, which we introduced recently in a somewhat di erent and less stable form [3].The idea that major contributions to the di raction pattern of an aperture in an opaque screen arise at the sharp material discontinuities at the edges of the aperture is not new: both the Maggi-Rubinowich boundary di raction wave theory and the rigorous theory of di raction by the edge of a perfectly conducting screen describe the di racted eld as a superposition of a geometrical wave and a wave generated at the edge [4]. Considering more general di ractive elements, in